Programming Model
MSC8144E Reference Manual, Rev. 3
Freescale Semiconductor
26-75
Direction fields in the header. The channel accesses the first data parcel by starting at a location
given by a POINTER value, and accessing a number of bytes given by a LENGTH or EXTENT
value. Subsequent data parcels may be accessed by starting where a previous data parcel ended,
or by starting at a different POINTER location. The LENGTH or EXTENT used with any
POINTER may be from the same pointer or from a different pointer in the same descriptor.
Although the EXTENT field exists in each pointer of the SEC descriptor, only the EXTENTs in
pointers 3, 4, and 5 are currently used. If Extend Address Enable is set (1), then the four EPTR
bits are concatenated with the POINTER field to form a 36-bit pointer address.
Examples of special cases include:
Sometimes, a descriptor field may not apply for the requested service. With seven
pointers, it is possible that not all these pointers are required to specify the input and
output parameters. (Some operations, for example, do not require context.) Where a
particular pointer is not used, all fields in that pointer should be cleared (0).
Some descriptors involve more than seven parcels of input and output data. In these cases,
it is necessary to use one POINTER field to address a sequence of data parcels.
LENGTH and EXTENT fields normally specify the sizes of data parcels. In some cases,
however, the POINTER field is zero, and the LENGTH and/or EXTENT fields simply
specify values to write to an EU.
The J bit in each pointer is used to enable the scatter/gather feature. If a data parcel to be
read or written by SEC is in one contiguous block of memory locations, then the
scatter/gather feature is not needed. In this case, set the POINTER to point directly at the
Table 26-9. Pointer Field Definitions
Bits
Name
Description
Settings
63–48
LENGTH
Length
The use of this field depends on the Descriptor Type and
Direction in the header. A value of zero causes the channel
to skip this pointer.
A number of bytes in the range 0
to 65535.
47
J
Jump
Determines whether to jump to a link table whenever the
POINTER field in this same field is used.
0
The POINTER field points to
data.
1
The POINTER field points to
a link table, and scatter/
gather is enabled.
46–40
EXTENT
Extent
A number of bytes in the range 0 to 127. The use of this field
depends on the Descriptor Type and Direction in the header.
39–36
—
Reserved
35–32
EPTR
Extended Pointer
Concatenated as the top 4 bits of the pointer when EAE is
high (see the EAE bit in Table 26-1).
31–0
POINTER
Pointer
A memory address.
Содержание MSC8144E
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Страница 520: ...MSC8144E Reference Manual Rev 3 12 68 Freescale Semiconductor DDR SDRAM Memory Controller ...
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